A typical RF system, for example a radar system capable of simultaneous transmission and receiving simultaneous transmit and receive radar (“STAR”), includes a transmitter for transmitting, for example, pulse modulated waves at a variety of different frequencies. A duplexer is coupled to the output of the transmitter to allow for bi-directional communication over a single path. A receiver is also coupled to the output of the duplexer for receiving the transmitted signals at the same time intervals as the frequencies of the transmitted signals. The duplexer isolates the receiver from the transmitter while permitting them to share a common antenna. Accordingly, the duplexer needs to operate in the frequency band used by the receiver and transmitter, provide adequate rejection of transmitter noise occurring at the receiver, operate at or less than the frequency separation between the transmitter and receiver, and prevent receiver desensitization.
However, transmit leakage (RF current and/or voltage) signal through isolation and antenna port reflection paths and mutual coupling from adjacent antenna elements in a phased array radar overloads the receive path generating noise in the RF receiver, resulting in inaccurate readings. This leakage or feedthrough signal comprises an unpropagated portion of the transmitted energy which is fed directly to the RF receiver resulting in saturation of and therefore degradation of the receiver.
In many cases, the received signal may include a leakage component when the transmit signal is reflected from non-target objects and the receiver receives a portion of the bounced transmit-signal, or when the receiver receives a portion of the transmit signal directly from the transmitter.
In order to cancel the leakage signal in the radio frequency (RF) path for a simultaneous transmit and receive application, filter coefficients must be calculated in real time and a cancellation signal must be adapted in real-time based on those coefficients to allow better cancellation. Also in order to meet radar specifications, very small latency is allowed on the adaptive filtering for the transmit line.
Some current techniques perform the cancellation via an analog feedback. However, the problem with the analog feedback is that the analog circuitry adds additional distortion to the cancellation signal which needs to be subsequently adapted out of the system. Fast digital control of the adaptive algorithm is not possible, and analog coefficients must often be calculated off line. In general, the slower the adaptive loop of the process, the less cancellation the process can achieve, because the change rate of the signal to which the system is adapting dictates the cancellation that can be achieved. For example, if the signal is changing significantly faster than the adaptive loop, cancellation is limited. Moreover, off-line computation of coefficients is not adaptive at all and does not address dynamic signals or dynamic environments well.
In general, current techniques to achieve a leakage cancellation are based on three basic methods: i) the cancellation is done in the analog domain where the equalization filter is applied in analog with a tapped delay line mechanism on the transmitted signal and the resulting delayed and amplitude equalized version of the transmitted signal, or cancellation signal with an inverse of the leakage content inside the received signal, is added to the received signal; ii) the cancellation is done in the analog domain where cancellation signal is created in the digital domain by passing the transmit waveform through the equalization filter and sent to a digital to analog converter to be added to the received signal to cancel the leakage in the analog domain; or iii) the cancellation is done in the digital domain where the equalization filter is applied in the digital domain on the transmitted waveform, and the digital cancellation signal and digital received signal are combined in the digital domain. However, all of these approaches suffer from two limitations. The first is that they typically need to accommodate several sampling rates, which complicates the system design. The second is that they produce undesirably large delays, which makes them unsuitable for certain high-speed (and real-time) applications. Large delays in the leakage cancellation signal results in substantial “blind spots” in certain radar applications, due to slower generation of adaptive parameters slower adaptations of the filter coefficients.